Techniques for applying a radiation measurement apparatus to nuclear medicine include a single photon emission computed tomography (hereinafter referred to as “SPECT”) using a gamma camera. The SPECT measures the distribution of a compound including a radioisotope and provides a tomographic plane image. The mainstream type of the conventional SPECT system is a type of apparatus formed by combining a scintillator consisting of one crystal plate with a plurality of photomultiplier tubes. The position of a radiation source in this type of SPECT system can often be calculated by, e.g., centroid calculation. However, the maximum resolution achieved by such calculation is approximately 10 mm, which is not sufficient to satisfy the requirements of the clinical setting.
In response, a pixelated detector extremely advantageous for enhancement of spatial resolution has been developed. Some pixelated detectors are composed of a scintillator, and other types are composed of a semiconductor. A positional signal of the radiation source acquired by any type of detector is on a small detector basis, in other word, on a pixel basis. The intrinsic spatial resolution of the detector is determined by the pixel size. Detectors with a pixel size of 1 to 2 mm have been developed. Since resolution of 10 mm or below has been achieved, detectors have been significantly improved.
New image reconstruction methods for a tomographic plane have been developed and improved to contribute to resolution enhancement. The Filtered Back-Projection (FBP) method and successive approximation methods without resolution recovery (such as the Maximum Likelihood Expectation Maximization (MLEM) method, and the Ordered Subset Expectation Maximization (OSEM) method). Recent years have seen development of a successive approximation method with a resolution recovery feature. This method enables reconstruction of a tomographic plane considering the geometry of a collimator or detector, attenuation of a gamma ray by the object, a scattered ray, and other physical factors.
Types of SPECT systems include entire body capturing apparatuses whose capturing range is wide enough to cover the entire body, dedicated cardiac and brain capturing apparatuses specialized for the brain and the heart, and dedicated cardiac capturing apparatuses specialized for the heart. Most widespread SPECT systems are entire body capturing apparatuses that can perform bone imaging by capturing the entire body. However, the quality of brain and heart images captured by an entire body capturing apparatus is inferior to the quality of images captured by, e.g., dedicated cardiac and brain capturing apparatuses specialized for capturing only the brain and the heart. Accordingly, there is a call for a SPECT system that can not only perform entire body capturing, but also capture images of the heart, the brain, and other organs whose quality is as good as the image quality of dedicated cardiac and brain capturing apparatuses.
PTL 1 and PTL 2 disclose the technique for capturing, e.g., SPECT images using a small-area radiation detector. For example, PTL 1 discloses that the detector is moved in the tangential direction of the rotational path of the detector so that the region of interest of the capturing target (e.g., the heart) is in the capturing range of the detector at each rotational position of the detector. Also, PTL 1 discloses that by changing the position of the detector between the first rotation and the second rotation in the tangential direction of the rotation path of the detector, the region of the heart is contained in the capturing range of the detector. Also, PTL 2 discloses that each of a plurality of small-area detectors is disposed to cover a region outside the capturing range of another region, whereby the entire body of the object is captured.